1. Field of the Invention
The present invention relates to a fiber-reinforced foamed material and a method of producing it. More particularly, the present invention relates to a fiber-reinforced foamed material which exhibits high resistance both to impact and flexing, as well as excellent shape maintaining characteristics and which is produced by allowing a foamable material capable of forming a highly rigid foamed body to foam together with specific composite fibers and/or specific composite yarns as reinforcers.
Hitherto, foamed materials such as those of polystyrene and phenol resin, which generally exhibit small distortion against external force and, hence, superior shape maintaining characteristics and dimensional stability, have been widely used as packaging materials for packaging various household electric appliances and precision instruments, materials of cooler boxes and heat-insulating architectural materials.
These known foamed materials, on the other hand, generally exhibit small resistance to stresses caused by application of an impact, bending or flexing. Thus, these foamed materials are liable to be cracked or bent even by a slight deformation, often resulting in destruction of the structures made from such foamed materials. For these reasons, the known foamed materials have been considered as being unsuitable for use as materials which are expected to undergo strong impact or deforming forces caused by bending or torsion. Thus, the known foamed materials have a limited use despite the above-mentioned advantages.
For instance, foamed polystyrene, which is widely used for packaging household electric appliances cannot be used alone since the packaging material tends to be broken by an impact to allow damaging of the contents or to generate dust or fractions of the foamed material during transportation. In order to obviate such problems, it has been a common measure to use another material such as a corrugated board together with the foamed material such that the formed material is coated and reinforced by the other material, or to increase the thickness of the foamed material. All these countermeasures require laborious steps in the production process and raise costs for transportation and storage due to an increase in the size and weight.
It has also been proposed to use a composite material in which a polystyrene foamed material is reinforced with another plastics material. Such a composite material, however, is generally too hard, heavy and expensive and, hence, is not preferred.
Methods have also been proposed in which a foamed material is reinforced with fibers mixed therein, as disclosed, for example, in JP-C-47-28097 and JP-A-48100471.These methods, however, cannot provide a substantial reinforcement effect because of insufficient affinity or integration between the reinforcement fibers and the foamed material. Thus, these methods also are still unsatisfactory.
Referring now to foamed materials of a phenol resin, attempts have been made to apply this type of material to architectural materials, since this type of material exhibits high resistances to heat and fire, in addition to the excellent shape maintaining characteristics and dimensional stability mentioned before. Foamed materials of phenol resins, however, are generally fragile and easily collapsible, which makes this type of material difficult to work and finish. In order to overcome this problem, measures have been taken such as laminating paper sheets on obverse and reverse sides of the foamed material or using a honeycomb structure with cells in which the resin foams. Such measures also are laborious and expensive. In addition, dropping of edges and fracturing into powder are liable to occur with this type of material during nail driving and cutting at construction sites. Thus, the aforesaid problems still remain unsolved.
Thus, attempts have been made to improve mechanical performance or shape maintaining characteristics, as well as dimensional stability, by strengthening the foamed material with fibers. Unfortunately, however, these attempts could not provide a satisfactory reinforcement effect due to insufficient integration between the foamed material and the fibers after the foaming. This is attributable to the fact that the temperature at which the fibers can be thermally deformed or become molten is generally much higher than the temperature at which the resin is allowed to foam. It would be possible to use a fiber which is thermally deformed or molten at a temperature below the foaming temperature. Such a fiber, however, cannot make any contribution to the reinforcement because the fiber itself can no longer have strength after the foaming.
Low-rigidity polymers such as polyethylene and polypropylene exhibit high resistances to bending and torsion by virtue of their excellent stretchability. A composite material having improved strength and dimensional stability would be obtainable by reinforcing such low-rigidity polymer with fibers. The low rigidity of the foamed material, however, allows an easy deformation of the foamed material. In such a composite material, the foamed resin is first deformed to leave stresses which are borne by the fibers. Such a composite material, therefore, cannot exhibit superior shape maintaining characteristics and dimensional stability, and no appreciable reinforcement effect is produced.
The behaviour of this type of material under a flexural stress will be described by way of example. When a bending force is applied to a material, tensile stress is generated in one side while compression stress is produced in the other side. A foamed material under a flexural stress, when the rigidity is low, is first buckled at the compressed side. Thus, the reinforcement effect of the fibers is developed only after such a buckling. It will be seen that the fibers cannot make any contribution.